There are always two threads - using this month for example:
"
Halloween 2009" - the first post is the description of this months contest,
and this is the thread where you submit your picture.
"
Halloween 2009 Chat " is the other thread, and like it says, it's there for people to discuss the entries.
That way the "Halloween 2009" thread only contains pics.
Finally, at some point near the end of the month, Yoshismom ( she's a mod who oversees the contest ) will start a Third thread,
called something like "
Halloween 2009 Voting" which is simply a poll.
That poll lists all the entries and you can cast your vote at that time.
Clear as mud now, isn't it ?